Comparison of Irrigation Management Strategies to Optimize Wine Grape

The third year of a four-year study was conducted in a Cabernet Sauvignon vineyard at J.
Lohr Winery, in the Paso Robles area during 2004. Treatments included three irrigation
strategies: sustained deficit irrigation (SuDI – where vines are irrigated at some fraction
of vineyard water use throughout the season), regulated deficit irrigation (RDI ? where
vines are deficit irrigated as some time during the growing season) and depletion of soil
moisture (water is depleted in the soil profile until a critical value of vine water status is reached and an irrigation event then takes place). Applied water amounts at various fractions (0.375, 0.56, 0.75 and 1.12) of estimated ETc were included in each of the
irrigation strategies, with the exception of the soil water depletion treatment. The PRD irrigation technique was not used in 2004 in this portion of the trial as it was not shown to differ from the SuDI technique the previous two years. However, a separate study comparing the following three irrigation treatments 1.12 SuDI, 0.56 SuDI and 0.56 PRD was continued in 2004.
Vine water status was monitored throughout the growing season. The results indicated
that the leaf water potential of vines irrigated a specific fraction of estimated ETc were
similar regardless of irrigation management technique. For example, if the vines were
irrigated at 0.375 times ETc, midday leaf water potential was similar regardless if
sustained deficit irrigation (SuDI) or regulated deficit irrigation (RDI) was being used at
the time the measurements were being made.
This study also included RDI as an irrigation management technique. The results
indicate that deficit irrigation between berry set and veraison and then irrigating at greater applied water amounts thereafter, was not as good as deficit irrigating throughout the growing season with regard to berry size. Deficit irrigation from veraison to harvest was as beneficial as deficit irrigation between set and veraison in reducing berry size the third year of the study. Berry size of vines irrigated only once every two weeks was similar to that of the 0.357 ETc irrigation amount using SuDI. This may indicate that this less precise method would be useful in reducing berry size. Lastly, as applied water increased yield increased. Vines irrigated at 0.375 of ETc, regardless of irrigation technique, reduced yields by approximately 50%. Small lot wines were made at UC-Davis of all nine, irrigation treatments used in 2004. These wines have not been analyzed but will be
evaluated in the upcoming year.